1. Field of the Invention
The present invention relates to a capacitive sensor device.
2. Description of the Related Art
A capacitive sensor device for detecting various physical quantities such as a pressure, an acceleration, a vibration, and'a sound pressure is known. This capacitive sensor device includes a pair of sensor capacitors. Electrostatic capacitances of these sensor capacitors are equal to each other in a state where the physical quantity is not applied and change in an increasing and decreasing relationship conflicting with each other in accordance with application of the physical quantity. Thus, the physical quantity applied to the capacitive sensor device can be detected with accuracy by converting a difference between the electrostatic capacitances of theses sensor capacitors into voltage.
The capacitive sensor device is ideally manufactured so that the electrostatic capacitances of the pair of sensor capacitors are equal in a state where the physical quantity is not applied. However, because there are manufacturing tolerances in these sensor capacitors, even in a state where the physical quantity is not applied to the capacitive sensor devices, there may be offset (mismatching) in the electrostatic capacitances of these sensor capacitors. It is difficult to distinguish the offset of the capacitances that exist irrelevant to the application of the physical quantity from a difference in the electrostatic capacitances caused by the application of the physical quantity in the output voltage of the capacitive sensor device, and it may cause a detection error of the physical quantity. Thus, a technique for compensating an offset of electrostatic capacitances of a pair of sensor capacitors has been developed.
Japanese Unexamined Patent Application Publication No. 2008-64742 discloses a technique for compensating an offset of electrostatic capacitances of a pair of sensor capacitors. In this technique, a plurality of compensating capacitors is formed on an element same as the pair of sensor capacitors. Then, in accordance with the actual offset of the electrostatic capacitances of the sensor capacitors, the electrostatic capacitances of the pair of sensor capacitors are adjusted to be equal to each other by cutting a wire in the compensating capacitors by a laser trimming or by adding a wire by bonding pad.
When the capacitive sensor device is used for a long term, the electrostatic capacitances of sensor capacitors may change across the ages. If the electrostatic capacitance of each of the pair of sensor capacitors changes across the ages, the offset of the electrostatic capacitances may also change. In the above-described technique, the electrostatic capacitors are adjusted irreversibly by cutting or adding wire. Thus, although the offset of the electrostatic capacitances at an adjusting stage in a manufacturing process of the capacitive sensor device can be compensated, it is difficult to compensate the offset when the offset of the electrostatic capacitances changes across the ages after that.